Dear friends.
Is it true the HVAC sys' in this car should hold a vacuum when the car is shut off ? That is, at the a/c ...My problem is no vacuum control to any distribution vents.....A/c works with correct pressures and blows cool air but obviously not COLD because the Recirc' door is not in Recirc'. AUTO fan works, and OFF shut sys' off . There is 23 inches of vacuum at the BROWN hose prior to the fire wall. Aux' fan comes on at 290 psi hi side. I 'think' the control button head is faulty but on reflection why would system work apart from being in DEFAULT....Checked all the iATN achives but no specifics...would like to get some feedback on this please...
regards Don at DKV Euro in Purcell Oklahoma.

The ACC system won't hold a vacuum for very long when the engine's off. There's a reservoir, but it's more to tide the sytem over during driving transitions, I think.

There's a little filter about 3-4 inches up along the small line at the vacuum pump. The ACC system will act strangely if the filter is clogged. I don't know if it can be cleaned but it's easy to replace or remove for testing. If that doesn't do it, then I'm pretty sure it's bad actuators in the ACC system.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD

I suppose I should be more specific...The system works fine apart from the air distribution doors are not moving when inputting different buttons....The system is blowing MAX fan, Auto fan and LOW fan just fine....I cannot find any vacuum schematics for the HVAC.....The compressor is functioning okay....Freon pressures are text book.......BUT, I seem to be missing VACUUM CONTROL to the actuators.....Sucking on the BROWN vac' line through the fire wall, holds vacuum......If the compressor was not working I would be thinking Klima relay or OVP Relay.....#5 fuse shows Batt+ voltage both sides....The Push Button assembly is not wanting to disassemble from the mother board on the bench. and I have no wish to break it.....all visable vac' hoses are on their respective ports, but I do not see the SUPPLY vacuum hose to the electro/vacuum soleniod control module.......
So this is where I am, and thank you SIXTO for your reply...BTW, There is no filter ajacent the vacuum pump, just large bore BLACK pipe to breke servo and SMALL bore hose to emmissions tee.
Thank you people.....
Don at DKV Euro in Purcell Oklahoma.

The brown tube might be for the engine stop switch. I believe the line to the ACC system is red with a green stripe. Follow it from the yellow check valve ahead of the brake master cylinder. On the cabin side, one line goes inside, the other line goes to a reservoir somewhere under the fuse box.

If that line holds a vacuum, check if the source provides a vacuum with the engine running. I don't know what it should be but 14" Hg is reasonable.

Then check if the distribution block on the right side of the center console is working. A functional diagram is available in the MB W126 CD manual but it's not necessary for finding faults since it's easy to follow vacuum lines to their actuators.

The easiest actuators to get to are the recirc actuators behind the glove box. Pry off the center posts of the 7 clips holding the glove box in place. Pry off the glove box light and thread it through the hole. Pull out the glove box and you should see a couple of pink actuators each with two vacuum ports. To the extent that you can get a MightyVac hose on the vacuum ports, see if they'll hold a vacuum. The actuators work in parallel so you can test the chambers in pairs if that's easier. If you reach to the left of the glove box near the top of the dash you'll feel the defrost vent actuator (or closer since default is open). It's the other dual chamber actuator in this system. Test that one as well.

That will tell you if those actuators are good or bad.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD

Well, tried that and the system is operating between the 'ALL vents' button and the 'FACE vents and floor' button, you can feel the air blowing more out of the FACE vents as you change to 'Face' from 'ALL'. So I seem to have vacuum, but, the Recirc' door still is not moving, there is no vacuum to either of the twin Recirc' vacuum actuators from the supply tube(s).....to open or close the Recirc' door...(I already have the glove box out.)
Now, I refilled the system (Freeze 12.) to be sure I have pure gas in it and went on road test.....Now, this is with the Recirc' door jammed into the Recirculating position with the blower on HI and wheel set to MAX cool.......after around 4 miles the vent temp was 52 C with ambient at 92C and Humidity 60% road speed 60 mph at 2,500 rpm....when the sun went behind a cloud at one point the temp momentarily dropped to 49C.....I feel this system is not getting the job done, and my own '92 124 300e gets vent temp's of 39 and lower !!!! (R12) Hurts your knuckles on a long drive, this 126 ain't gettin' it !
Thanx again for your help , I am hoping there may be something to this I am missing by standing to close to the problem.....
Regards...Don at DKV Euro in Purcell Oklahoma

BTW....I removed the blower and checked for obstruction in the A/C evaporator core, fairly clean for this year vehicle, I've had them come in with low air volumn output, turned out that evap' totally blocked with leaves and tissue paper.
Don

If you have no vac at the recirc servos, you have either a faulty switchover valve, a leak, or loose hose, or a bad PBCU.

The system MUST hold vac indefinitely on the GREEN plastic line from the engine compartment. If it leaks down, engine on or off, you have some leaks. Vac will drop slightly when servos are activated, but must not continue to drop when they are pulled down.

52 F cooled air is probably correct -- the evaoprator temp sensor will cut the compressor off when the evaporator gets to 35 F to prevent freezing. Don't forget that if the monovalve bellows are bad, you will also have heat in the heater core at the same time.

Pull a vac on the supply line and fix any leaks that show up before condeming the PBCU.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Okay folks, here's the latest....
After dark test: Run a/c on MAX, center vent air temperature finally dropped to 48 F (Please read 'F' for any previously posted temperature from interior a/c vents. Seems I was stating Celsius. tut tut !) at this temp' the compressor shut off until vent temp' reached 53F and recycled down to 48F again....continued this for duration of test...HOWEVER ..it seems that the system takes too long to drop temperature on start up. My '92 300e goes straight to cold after about 2 min's on a hot day...this SDL takes 10 to 15 min's driving to get below 60F...Fan spins good, and no obstructions...I know I am probable flogging a dead horse here, but it bugs me when these HVAC's do wiered stuff.....gotta get to the bottom of this somehow.....I disconnected the MONO VALVE whilst testing and the vent temperature climbed to 83F, reconnected Mono and temp dropped slowly to 48F once more. This all with the Recirc' door proped in the recirc' position......Well, will test vacuum tomorrow as per last post and see if I can find that elusive filter in the vacuum line....after that I will see about changing out the expansion valve......Thank you everyone for your help, I will post a fix ASAP......
Don.

The default state of the monovalve is open. When you pull the plug, you allow full coolant flow to the heater core. Possibly without the circulation pump, but it'll get pretty darn hot.

I didn't realize that you are a tech. I hope I didn't come across as condescending. Not that I would be to an anybody.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD

Pull the monovalve (two screws, in plastic, so be careful to get back into the existing thread going back in) and look at the rubber diaphram. If it is wrinkled or has sharp creases in it, replace it, it's either leaking or not closing properly, and hot coolant is leaking past the valve to the heater core.

You should be getting cooling pretty fast -- both the 300 s blow cool air right away, but it does take several minutes at 45-50 mph for it to get cold.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!